As a result of an increase in requirements for more energy saving as seen in recent years, an attention is being focused on, for example, an electric vehicle driven by an electric motor, and a hybrid vehicle driven by a an electric motor in combination with a reciprocating engine in the automobile field. In these vehicles, a semiconductor device with a power semiconductor element mounted therein is used in order to convert DC power supplied from an onboard battery into AC power to thereby drive a high-output electric motor. With the semiconductor device described as above, a heat value due to energization of the power semiconductor element is large, so that it is required to take heat-release properties into consideration.
A technology described in Japanese Unexamined Patent Application Publication No. 2005-136018 is well known as a technology for enhancing the heat-release properties of the semiconductor device with the power semiconductor element mounted therein. In Japanese Unexamined Patent Application Publication No. 2005-136018, there is described a semiconductor device made up by sequentially stacking a lower-side heat sink, a first solder layer, a semiconductor element, a second solder layer, a heat sink block, a third solder layer, and an upper-side heat sink to thereby cause the semiconductor element to be electrically connected between the upper-side heat sink and the lower-side heat sink via the first to third solder layers, respectively.